Question 985436
let's start with the equation
let C represent the total bill, P be cost of parts, M be mechanic charge per hour, H be total hours on job, CL be cost of labor (M * H)
C = P + CL
we are given C = $340, P = $220, M = $40, therefore
340 = 220 + (40 * H)
subtract 220 from both sides of =
(40 * H) = 120 
divide both sides of = by 40
H = 3
The mechanic works 3 hours on the car
The total bill varies directly as the cost of parts and the cost of labor.